Electromagnet and assembly

ABSTRACT

An electromagnet for a Magnetic Resonance Imaging (MRI) apparatus. The electromagnet includes a coil configured to generate a magnetic field. The coil has a first axially outer surface, and a support element configured to mount the coil in the MRI. The support element is bonded to the first axially outer surface of the coil.

The present disclosure relates to an arrangement for mounting anelectromagnet suitable for applications in Magnetic Resonance Imaging.

In particular the disclosure relates to an electromagnet and anelectromagnet assembly for a Magnetic Resonance Imaging apparatus.

BACKGROUND

In Magnetic Resonance Imaging a plurality of electromagnets comprisingsuperconducting coils is energised to generate a magnetic field which isboth strong and spatially confined. This causes interaction between theelectromagnets, thus subjecting them to electromagnetic loads. Moreover,transportation and installation may also subject the electromagnets toloads. In order to maintain the desired performance of theelectromagnets, such loads need to be managed by the means used forsupporting the coil.

A known means for supporting electromagnets involves amechanically-supported journal into which the coil is wound. Thisarrangement, however, may not sufficiently restrain relative movement ofthe journal and the electromagnets.

Another known means for supporting electromagnets involves a tensilesupport wrapped around the electromagnets. This arrangement may notprovide sufficient rotational restraint to effectively managetransportation loads. Additionally such an arrangement may be difficultto configure to adequately achieve small tolerances required forspatially locating the electromagnets. Any deviation from an optimalrelative position of the electromagnets may result in local stressconcentration detrimental to performance.

Hence an arrangement which allows for accurate and secure mounting of anelectromagnet of a MRI device is highly desirable.

The invention of the present application has certain features in commonwith the invention of International patent applicationPCT/EP2018/075940.

SUMMARY

According to the present disclosure there is provided an apparatus andmethod as set forth in the appended claims. Other features of theinvention will be apparent from the dependent claims, and thedescription which follows.

Accordingly there may be provided an electromagnet 100 for a MagneticResonance Imaging MRI apparatus. The electromagnet 100 may comprise acoil 110 for generating a magnetic field, the coil 110 having a firstaxially outer surface 112, and a support element 120 for mounting thecoil 110 in the MRI. The support element 120 may be bonded to the firstaxially outer surface 112 of the coil 110.

Thus the bonded support element provides a means for securing the coilto a support structure and, thereby, inhibit axial and rotationalmovement of the electromagnet.

The support element 120 may be configured for being mechanicallyfastened to a support structure 200.

The support element 120 may comprise a bonding face 122 by which thesupport element 120 is bonded to the first axially outer surface 112.The bonding face 122 may be configured to have a reduced stiffnesstowards its periphery and a higher stiffness towards a central area ofthe bonding face.

Thus the support element is configured to reduce a stress concentrationwhich may arise at the periphery of the bonding face in response to aforce acting on the electromagnet.

A plurality of support elements 120 may be bonded to the first axiallyouter surface. The plurality of support elements 120 may be equallyspaced around the first axially outer surface 112.

According to other examples, the plurality of support elements may beirregularly spaced. That is to say, the plurality of support elements120 may be unequally distributed around the first axially outer surface112.

The coil 110 may comprise a second axially outer surface 114 opposite tothe first axially outer surface 112. A further support element 120′(FIG. 14) may be bonded to the second axially outer surface 114.

The electromagnet 100 and the support element 120 may be bonded using anadhesive suitable for cryogenic applications.

The coil 110 and the support element 120 may be resin-impregnated toform a monolithic structure.

According to another example there may be provided an electromagnetassembly 10 for a MRI apparatus, comprising: the electromagnet 100according to the present disclosure, and a support structure 200comprising a support plate 220, wherein the support element 120 ismounted to the support plate 220.

An extension tube 310 may be fitted around a pin 300 for making a pinnedconnection between the support plate 220 and the support element 120.The extension tube 310 may be configured to abut against the supportplate 220 and the pin 300 to maintain radial compression on the supportelement 120 when the coil 110 is in its superconducting state.

Thus the extension tube is configured to maintain compression in a casewhere the support element 120 has a greater coefficient of thermalcontraction than the pin 300.

The support plate 220 may be configured to press the support element 120and the further support element 120′ against the coil 110 when the coilis in its superconducting state.

Thus the support element 120 and the further support element 120′, ifany, are configured to secure the coil. In use, the support element andthe further support element may cooperate with the support plate topress against the first axially outer surface 112 of the coil 110 andthe second axially outer surface 114 of the coil 110.

The support element 120 and the support plate 220 may be configured tomake a pinned connection for securing the electromagnet 100 to thesupport structure 200.

According to another example there may be provided an MRI apparatuscomprising the electromagnet assembly 10 as described above.

According to another example there may be provided a method ofmanufacturing an electromagnet for an MRI apparatus. The method maycomprise winding a superconductor wire into a mould to form a coil 110,arranging a support element 120 against a first external surface 112 ofthe coil 110, impregnating the coil 110 and the support element 120 witha thermosetting resin, and removing the coil 110 and the support element120 from the mould.

The method of manufacturing may comprise placing a further supportelement 120′ against a second external surface 114 of the coil 110, thesecond external surface 114 being opposite to the first external surface112 of the coil 110.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the present disclosure will now be described with referenceto the accompanying drawings, in which:

FIG. 1 is a schematic perspective view of an electromagnet assembly;

FIG. 2 shows a radial cross-section of the electromagnet assembly ofFIG. 1;

FIG. 3 shows a radial cross-section of an electromagnet according toFIGS. 1 and 2;

FIG. 4 shows a tangential cross-section of the electromagnet of FIG. 3;

FIG. 5 is a radial cross-section of the electromagnet assembly;

FIG. 6 shows a tangential cross-section of the electromagnet of FIG. 5;

FIG. 7 shows a radial cross-section of a first variant of theelectromagnet assembly;

FIG. 8 shows a tangential cross-section of the electromagnet assembly ofFIG. 7;

FIG. 9 is a radial cross-section of an electromagnet according to FIGS.7 and 8;

FIG. 10 is a radial cross-section of a second variant of theelectromagnet assembly;

FIG. 11 shows a tangential cross-section of another electromagnet;

FIG. 12 shows a tangential cross-section of yet a further electromagnet;

FIGS. 13A, 13B show further variants of the electromagnet assembly ofthe present invention; and

FIG. 14 shows a perspective view of further example of an electromagnetaccording to the present disclosure.

DETAILED DESCRIPTION

The present application is concerned with an electromagnet suitable forbeing mounted in an electromagnet assembly which restrains theelectromagnet axially and circumferentially, thereby preventingdistortion of the electromagnet arrangement along or around its assemblyaxis A-A (described below).

FIG. 1 shows a schematic illustration of a perspective view of anelectromagnet assembly 10 according to the present disclosure.

In use, the electromagnet assembly 10 forms part of a Magnetic ResonanceImaging (MRI) apparatus (or ‘scanner’). For such a purpose theelectromagnet assembly may be contained within a housing which, inoperation, contains an inert gas as a coolant, for example helium. Hencethe housing forms a cryogen vessel, which enables the electromagnetassembly to be cooled to sufficiently low temperatures to optimiseperformance.

The electromagnet assembly 10 is generallyrotationally/circumferentially symmetrical, defining an assembly axisA-A, a radial direction and a circumferential direction. Accordingly,“axial” refers to a direction parallel to the assembly axis, while“radial” refers to a direction perpendicular to the assembly axis, and“circumferential” refers to a direction perpendicular to both theassembly axis and the radial direction around the assembly axis A-A. Theelectromagnet assembly extends axially (or “lengthwise”) along theassembly axis.

The electromagnet assembly 10 comprises a pair of electromagnets 100 anda support structure 200 configured to carry the electromagnets.

The support structure 200 is arranged to retain the electromagnets 100in a particular spatial (i.e. relatively spaced) arrangement, preventingeach electromagnet from moving along the assembly axis A-A or rotatingabout the assembly axis. For this purpose, the electromagnets aremechanically fastened to the support structure. According to the presentexample, the electromagnets 100 are mounted to at least two brackets201, 202 of the support structure. In alternative examples a differentnumber of brackets may be provided. For example, greater than threebrackets may be provided. The number of brackets used may be (at leastin part) dependent on the expected loads and the allowable deflection ofthe electromagnets.

FIG. 2 shows a cross-sectional view of the electromagnet assembly 10wherein the electromagnets 100 are mounted to the support structure 200.

According to the present example, the electromagnets 100 are configuredto generate a magnetic field which, in use, actively shields a magneticfield which is generated by a main magnet of the MRI apparatus. Eachelectromagnet 100 comprises a coil 110 (or ‘shield coil’) configured togenerate a magnet field.

Further, each electromagnet comprises a support element 120, which ismountable in the MRI apparatus. The support element is configured torestrain the coil against forces caused, for example, in response toelectromagnetic interaction between coils. For the present example it isassumed that electromagnetic interaction causes a force on each coilwhich is directed outwardly with respect to the electromagnet assembly10. That is, each coil experiences a force pressing the coil against itsrespective support element. Further, the support element also restrainsthe coil against rotational forces as may be caused, for example, duringtransportation.

The coil 110 is formed of a superconductor wire wound into an annularstructure. The coil is therefore essentially rotationally symmetrical.The coil may be described in terms of an axial direction, a radialdirection and a circumferential direction. When considered in thecontext of the MRI apparatus, these directions correspond to thosedescribed above in relation to the MRI apparatus as a whole. Inparticular, the coil possesses rotational symmetry about the assemblyaxis A-A.

FIGS. 3 and 4 show the coil 110 and the support element 120.

The coil 110 has a first axial end face 112 and, opposite thereto, asecond axial end face 114. The pair of axial end faces delimits anextent of the coil along the axial direction. Similarly, the coil has afirst radial surface 116 (or ‘inner radial surface’) and, oppositethereto, a second radial surface 118 (or ‘outer radial surface’). Thepair of radial surfaces delimits an extent of the coil along the radialdirection.

The support element 120 is fixed to the coil 110 and provides a meansfor mounting the coil in the MRI and, more specifically, to the supportstructure 200. The support element has a bonding face 122 which isbonded to the coil. The support element, or at least the bonding face,is configured to be bonded using an adhesive suitable for cryogenicapplications. According to the present example, the support element isbonded to the first axial end face 112 of the coil.

The support element 120 possesses a pair of mating surfaces 124, 126configured for being brought into abutment/contact with the supportstructure 200. According to the present example, the mating surfaces124, 126 are substantially flat so as to sit flat against acorresponding portion of the support structure 200.

According to the present example, in which the support element 120 isbonded to the first axial end face 122, the mating surfaces a separatedby a distance which is greater than the radial extent (or ‘radialthickness’) of the coil 110. Thereby the curvature of the coil may beaccommodated between the mating surfaces. Radial expansion of the coil,in response to energising, is also accommodated by suitably spacing themating surfaces.

The support element 120 is configured for mounting the coil to thesupport structure 200. Accordingly, the support element 120 comprisesmeans for mechanically securing to the support structure 200. Anysuitable means may be used such as, for examples, bolts, pins or othersuitable mechanical elements. In the present example the support elementcomprises an aperture 128 for receiving a pin and thus make a pinnedconnection with the support structure. By way of example, a pin 300 isshown in FIG. 13. However, for clarity, the pins (or other mechanicalelements) are not shown in FIGS. 2 to 12.

FIGS. 5 and 6 show the electromagnet 100 comprising coil 110 mounted inthe support structure 200. More particularly, FIG. 5 shows a radialcross-section of the support structure, the coil 110 and the supportelement 120, while FIG. 6 shows a corresponding tangentialcross-section.

The support structure 200 comprises a single support plate 220configured to receive the electromagnet 100. According to the presentexample, the support plate 220 includes a substantially flat section 222which matches the substantially flat mating surfaces 116, 118 of thesupport element. As explained with reference to FIG. 1, the exampleelectromagnet assembly 10 comprises at least two brackets 201, 202. Eachbracket comprises a single support plate 220.

The single support plate may have features formed into it or added to itto improve axial stiffness. The single support plate may be located onthe radially outer (A2) or radially inner (A1) sides of the coils. Thechoice of radially inner or radially outer sides of the coils may bedetermined by the expected coil deformation forces, and the requirementfor reducing the outer diameter of the magnet coil assembly. If thesingle support plates are located on the radially inner side of thecoils, then no radial space is taken up by the support plate. If thesingle support plates are located on the radially outer side of thecoils, then surrounding structures such as cryogen vessel or thermalradiation shield will need to be increased in diameter to accommodatethe support plates.

The support plate 220 may be flat or formed, to determine requiredstiffness, to manage stress and deflections where necessary to provideoptimum operating conditions for the superconducting coils.

According to the present example, where a pair of electromagnets 100 iscarried by the single support plate 220, each support plate extends froma first electromagnet 100 to a second electromagnet 100′.

When received on the single support plate 220, the support element 120is anchored to the support plate. Suitably an aperture 206 is providedin each support plate 220. For securing the support element, theaperture 128 of the support element 120 is aligned with the aperture 206of the support plate. A pin 300 (FIGS. 13A, 13B) is then fitted throughthe aligned apertures 206, 128, thus bringing the electromagnet assembly10 from a circumferentially unlocked configuration into acircumferentially locked/fixed configuration. Other mechanicalattachment means may be provided in place of the pin 300 to achieve thecircumferential locking function. In the circumferentially lockedconfiguration, relative movement between the support element 120 and thesupport structure 200 is restrained (or ‘inhibited’).

A bracing element 230 may be provided. The size and/or location of thebracing element is chosen to modify the stiffness of the support element220, where necessary, in order to accommodate expected loads acting onthe electromagnet assembly 10. Such loads may be caused by ferrousbuilding material present where the electromagnet assembly will beoperated (for example metal reinforcement rods or girders in floors andwalls). Other loads may be caused internally, particularly in responseto interaction between magnetic fields. More particularly, aninwardly-acting compressive coaxial force may be caused on eachelectromagnet 10. That is, the electromagnet assembly 10 may also beconfigured to operate in compression if the electromagnetic loaddirection is reversed. Sufficient bracing of the single support plate220 will be required to prevent buckling, and the bracing element(s) 230may be sized and/or located suitably.

For applications relating to Magnetic Resonance Imaging, it isconsidered convenient to assemble an MRI apparatus completely, or asmuch as feasible, before transportation to its destination. This maysave days or weeks of assembly time. Also, without requiring initialassembly and final assembly, this may reduce the amount of coolantrequired, thus saving precious resources resulting in a ‘greener’ finalproduct. During transportation of a fully or partially assembledapparatus, however, rotational loads may act on the coils 110. Ifunchecked, the forces can misalign or deform the coils and,consequently, adversely affect performance of the assembly.

However the provision of the bonded support elements 120 according tothe present invention provides axial restraint and rotational (or‘circumferential’) restraint of each electromagnet 100 relative to thesupport structure 200.

Additionally, during operation the electromagnetic interaction betweenthe coils 110 is restrained by the interaction of the support elements120 and support structure 200, thus preventing relative axial and radialmovement of the coils, thereby optimising performance of the device.

Hence the bonded support elements 120 of the present disclosure providesupport for the coils 110 against rotational (circumferential) loads toinhibit distortion or rotation of the assembly during transit and inoperation, thus increasing the chance of optimised performance of thefinal product.

The exemplary electromagnet 100 according to the present disclosure maybe manufactured using any suitable process of manufacturing.

An example process by which the electromagnet 100 may be manufacturedrelates to the known process of resin-impregnation. As part of thisprocess, superconductor wire is wound into a mould. The support element120 is arranged with the coil so formed, locating the support elementagainst an external surface of the coil. Where a plurality of supportelements are located on the external surface, they may be arranged withequal spacing.

The resulting structure is then impregnated with a thermosetting resinand the resin allowed or caused to set. Subsequently the electromagnetis removed from the mould as a monolithic structure.

According to the example process described, the step of forming the coilby winding superconductor wire is essentially unaffected by the lateraddition of the support element 120. That is, the coil is formed into ashape optimised for performance, essentially without consideration forbonding to or being carried by the support element.

The bonding between the coil 110 and the support element 120 may also beachieved using resin-impregnation, either at the same time as formingthe coil, or once the coil is formed. Resin-impregnation is known toprovide great structural strength to a coil, but according to thepresent disclosure said structural strength is used also for bonding thecoil and the support element. Notably, the bond may be reinforced, forexample through the addition of glass fibre to the resin, to provide abond sufficiently strong for the intended application. In particular,the bond may be made strong enough to withstand a tensile force whichmay be caused in response to a reversal of the electromagneticinteraction between the electromagnets 100.

Alternatively the support element 120 is secured to a resin-impregnatedcoil 110 in a separate manufacturing step. This may be achieved using anadhesive suitable for, where desired, cryogenic applications.

The prepared electromagnet 100 is mounted to the support structure 200to form the electromagnet assembly 10. This process includes locatingthe support element 120 to the support plate 220, aligning the aperture128 of the support element with the aperture 206 of the support plate,and fitting a pin 300 through the aligned apertures. The support element120 and the single support plate 220 are provided with appropriatefeatures to accept the pin 300, or to accept equivalent mechanicalfasteners where pins are not used. It is noted that shape-matching ofpins, particularly round pins, in corresponding apertures is efficientlyachievable to high accuracy. This provides for a predictable loaddistribution to ensure optimal performance of the electromagnets.Multiple pins may be provided for a single support element 120,according to loading conditions. The sizes and positions of the pins mayalso be selected according to loading conditions. The pins 300 should beclose fitting to the apertures 128, 206 to ensure reproducible stressdistribution. Other fastener types may be used to provide the equivalentmechanical function.

Particularly for applications relating to cryogenic applications, whichincludes Magnetic Resonance Imaging, the electromagnets 100 are cooledbelow a critical temperature at which the coils 110 enter asuperconducting state. For efficient cooling it is beneficial to exposea large surface area of the coils to the coolant. Conveniently, theelectromagnet 10 according to the present disclosure allows forrelatively small support elements 120 so that a relatively large surfacearea or each coil 110 remains exposed.

FIGS. 7, 8 and 9 show an alternative arrangement of the electromagnetassembly 10 which is configured to support particularly againstcompressive loads.

FIG. 7 is a radial cross-sectional view of the electromagnet assembly10, while FIG. 8 is a plan view of the electromagnet assembly, and FIG.9 shows a tangential cross-section of the electromagnet 100. Similar toFIGS. 5 and 6, FIGS. 7 and 8 are partial views in the sense that only aright-hand side of the electromagnet assembly 10 is depicted. That is tosay, the essentially identical left-hand side is omitted from thefigure.

According to the example shown in FIGS. 7 and 8, the single supportplate 220 extends only partway between the pair of electromagnets 100,100′. This is in contrast to the example of FIG. 5 in which the singlesupport plate extends all of the way from the first electromagnet 100 tothe second electromagnet 100′. Put another way, in the example of FIGS.7, 8 each electromagnet 100 is provided with its own single supportplate 220. A bracing element 230 is provided to each single supportplate 220. A rod 240, which may also be provided in the above example,extends between the bracing elements 230. The rod 240 may bemechanically mounted to the inner structure of the magnet assembly. Thesingle support plate 220 must be sufficiently strong to preventbuckling. This may be achieved by control of the material, shape and/orthickness of the single support plate 220.

The example of FIGS. 7, 8 and 9 also illustrates an alternative shape ofthe support element 120. According to this example, the bonding face 122possesses reduced stiffness towards the periphery delimiting the bondingface. That is to say, the support element 120 is formed to be moreflexible on its bonding face 122 towards a periphery of the bonding facethan towards a central area of the bonding face, the central area beingdelimited by the periphery. This arrangement is suitable for minimising“edge effects”. “Edge effects” may arise in response to stresses exertedon the coil 110, irrespective of whether compressive or tensile loadsare concerned. As regards a compressive load where the coil 110 ispressed against the bonding face 122, the edge effects may result in astress concentration at the periphery (or ‘edge’) of the bonding facewhich may cause damage to the coil or the support element. As regards atensile load where the coil is pulled along an axial direction away fromthe bonding face, a stress concentration at the periphery of the bondingface may cause damage to the bond between the coil and the bonding face.Reducing edge effects will thus also reduce the likelihood of failure ofthe bond between the support element 120 and coil 100.

In further embodiments, the support element may be mounted axiallyinboard of the coil, thereby being configured for operation incompression. FIG. 16 shows such an arrangement, in views similar toFIGS. 5 and 4, described above. In the embodiment of FIG. 16, coil 110is provided only with axially inner support elements 120′. FIG. 17 showsanother view of such an embodiment, in a view similar to the view ofFIG. 6.

FIG. 10 shows a radial cross-sectional view of a part of an alternativeexample of an electromagnet assembly 10′ according to the presentdisclosure. According to the example depicted in FIG. 10, theelectromagnet assembly is configured to operate in compression. That is,in use the electromagnetic load direction acts to bring the coils 100,100′ together along the assembly axis A-A. To improve resistance tobuckling, the support plate 220 and the bracing element 230 are formedintegrally. The support plate 220 and the bracing element 230 maycomprise solid or hollow sections. For example the support plate 220 andthe bracing element 230 may be solid or hollow section extrusions.Additionally, the passage/cavity 215 corresponds to a local cut-outsized to accommodate the support element 120 only. This arrangementtherefore provides for a relatively large bracing element and arelatively small passage 215, providing a particularly high resistanceto buckling under compression. With suitable configuration and/or choiceof materials, for example those of the coil surface 112 and the bondingface 122, this example electromagnet assembly may also be operable intension, where the integral bracing element 230 supports the inducedloads.

FIG. 11 illustrates an alternative shape for the support element 120.According to this example, the support element has a generally trapezoidcross-sectional shape. The legs (or non-parallel sides) of the trapezoidshape are convex to further reduce stiffness towards the periphery.

FIG. 12 shows another alternative shape for the support element 120configured to reduce edge effects. According to this example, thesupport element comprises cutaway sections towards the periphery of thebonding face 122, which provides the periphery of the bonding face withgreater flexibility.

FIGS. 13A and 13B show alternative examples of an electromagnet assembly10″ according to the present disclosure. According to this example, theelectromagnet 100 is mounted to the support structure 200 such that acompressive load exerted by the pinned connection is maintained aftercooling the system below the critical temperature of the superconductorcoil 110. Where the support element 120 has a higher coefficient ofthermal contraction than the pin 300 or pins used to make the pinnedconnection, as the system is cooled compression cannot be maintained sothat the electromagnet is not fully secured. According to the presentexample, an extension tube 310 is fitted around the pin, wherein theextension tube (or ‘sleeve’) has a lower coefficient of thermalcontraction than the pin. The extension tube is configured to abutagainst the support plate 220 and the pin 300 to compress the supportplates against the support element even when cooled, and thus maintaincompression. FIG. 13A shows an example where the extension tube 310 islocated on an opposite side of the support plate 220 to the supportelement 120, while FIG. 13B shows an example where the extension tube310 is located on a same side of the support plate 220 as the supportelement 120.

FIG. 14 shows a perspective view of a variant of the electromagnet 100.In the examples described above, at least one support element 120 isbonded to the first axial end face 112. In other examples, at least onesupport element 120 may be bonded to each axial end face 112, 114 or,more generally, to each pair of opposing external surfaces of the coil.That is, a first support element is bonded to a first outer surface ofthe coil, a second support element is bonded to a corresponding locationof a second outer surface which is opposite to the first outer surface.According to FIG. 14, the first support element 120 is bonded to thefirst axial end face 112, while the second support element 120′ isbonded to the second axial end face 114.

It is considered convenient to provide a support structure configured tocompress the coil along the assembly axis A-A when the support structureand the coil are cooled and/or the coil is energised. For this purpose,the support structure 200 may be configured to possess greater thermalcontraction than the coil along the axial direction. Additionally oralternatively, the support structure and the coil are configured so thatan expansion of the coil on being energised results in the supportstructure pressing the support elements against the coil. Conveniently,the described arrangement may reduce misalignment of the coil on beingenergised and may improve durability of the bonding.

In relation to the examples where the support elements 120, 120′ arebonded to the axial end faces, the first support element is located at aparticular circumferential location on the first axial end face 112, anda second support element is located in on essentially the samecircumferential location on the second axial end face 114.

Where a support element 120 is bonded to a radial surface, the bondingface 122 of said support element is suitably curved, convex or concave,dependent on the particular radial surface to which it is mounted.Further, the support structure is suitably adapted for mounting anelectromagnet 100 having a support element or elements 120 on a radialsurface or surfaces 116, 118 (FIG. 3).

As illustrated for example in FIG. 15, structures similar to thosedescribed above may be employed in constructing coils that constitutethe inner (main) magnet of the MRI system. When used for the inner(main) magnet, it is preferable that the single support plate 220 shouldbe placed on the radially outer (A2) surface of the coils. This avoidsreducing the available inner diameter of the coils, ensuring that theopen bore of the final MRI system is as large as possible. Typically,the coils of the inner (main) magnet are, in use, in a state ofcompression, that they are subjected to an axial force which tends todrive them towards the centre of the magnet. For that reason, it may beappropriate to use a structure similar to that shown in FIGS. 16, 17 asdescribed above, where each coil 110 is provided with an axially innersupport element 120′. The other features of such as embodiment are asdescribed above with reference to other embodiments. FIG. 15 shows apartial cross-section, in which one-quarter of the magnet isrepresented. The remaining structure will be essentially symmetricalabout axis A-A and centreline C-C. In a preferred embodiment, theindividual coils are connected to a longitudinal compression supportplate by similar pin joints. The pin jointed structure minimises theinteraction between the coils and the remainder of the inner magnetstructure.

Attention is directed to all papers and documents which are filedconcurrently with or previous to this specification in connection withthis application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings) may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

The present invention differs from the invention disclosed inPCT/EP2018/075940 in that only a single support plate 220 is provided toeach coil. In PCT/EP2018/075940, two support plates are provided to eachcoil, one adjacent the respective axially inner surface and one adjacentthe respective axially outer surface. The present invention provides theadvantages that fewer components are required with attendant savings inweight and material cost. Furthermore, use of a single support plate, asprovided by the present invention, may allow smaller components such asthermal shield and outer vacuum container to be used, as support plates220 need not be provided radially outside of the outer coils, orradially inside the inner (main) coils. In turn, this may mean that theclear bore of the final MRI system may be enlarged by comparison toembodiments of the invention of PCT/EP2018/075940, or smaller diametercoils may be used for a given bore size, reducing material cost andweight of the magnet.

1-9. (canceled)
 10. An electromagnet assembly for an MRI apparatus,comprising: an electromagnet for a Magnetic Resonance Imaging (MRI)apparatus, the electromagnet comprising: an annular coil configured togenerate a magnetic field, the annular coil having a first axially outersurface; a support element configured to mount the annular coil in theMRI apparatus, wherein the support element is bonded only to the firstaxially outer surface of the annular coil; and a support structurecomprising only a single support plate located either on a radiallyouter or radially inner side of the annular coil, wherein the supportelement is mounted to the single support plate, and is mechanicallyfastened to the single support plate.
 11. The electromagnet according toclaim 10, wherein the support element comprises a bonding face by whichthe support element is bonded to the first axially outer surface, andthe bonding face is configured to have a reduced stiffness towards itsperiphery and a higher stiffness towards a central area of the bondingface.
 12. The electromagnet according to claim 11, further comprising: aplurality of support elements bonded, each only to the first axiallyouter surface, and equally spaced around the first axially outersurface.
 13. The electromagnet according to claim 10, wherein theannular coil comprises a second axially outer surface opposite to thefirst axially outer surface, and a further support element is bondedonly to the second axially outer surface.
 14. The electromagnetaccording to claim 13, wherein the annular coil and the support elementare resin-impregnated to form a monolithic structure.
 15. Theelectromagnet assembly according to claim 10, further comprising: anextension tube fitted around a pin configured to form a pinnedconnection between the single support plate and the support element,wherein the extension tube is configured to abut against the singlesupport plate or the pin, and to maintain compression on the supportelement when the coil is in its superconducting state.
 16. Theelectromagnet assembly according to claim 14, wherein the single supportplate is configured to press the support element and the further supportelement against the coil when the coil is in its superconducting state.17. The electromagnet assembly according to claim 15, wherein thesupport element and the single support plate are configured to form apinned connection for securing the electromagnet to the supportstructure.
 18. An MRI apparatus comprising the electromagnet assemblyaccording to claim 10.